With the rapid development of digitalization of information, digitalization in image processing is increasingly required. In digital cameras in particular, solid-state image pickup devices, such as Charge Coupled Devices (CCD) and Complementary Metal Oxide Semiconductor (CMOS) sensors, have been mainly provided on imaging planes instead of films.
In image pickup apparatuses including CCDs or CMOS sensors, an image of an object is optically taken by an optical system and is extracted by an image pickup device in a form of an electric signal. Such apparatuses may be used in digital still cameras, video cameras, digital video units, personal computers, mobile phones, PDAs, image inspection apparatuses, industrial cameras used for automatic control, and the like.
FIG. 1 is a schematic diagram illustrating a structure of an existing image pickup apparatus 1 and traces of light ray bundles. The image pickup apparatus 1 comprises an optical system 2 and an image pickup device 3. The optical system 2 includes object-side lenses 21 and 22, an aperture stop 23, and an imaging lens 24 arranged in order from an object side (OBJS) toward the image pickup device 3. The image pickup device 3 may be a semiconductor sensor such as a CCD and a CMOS sensor.
The object-side lenses 21 and 22 focus the image of an object before the aperture stop 23, and the imaging lens 24 focuses the image of an object after the aperture stop 23. The optical system 2 may be telecentric.
Telecentricity is a special property of certain multi-element lens designs in which chief rays for all points across the object or image are collimated. A chief ray is any ray from an off-axis object point which passes through the center of the aperture stop 23 of the optical system 2. The chief ray enters the optical system 2 along a line directed toward the midpoint of the entrance pupil, and leaves the system along a line passing through the center of the exit pupil. For example, telecentricity occurs when the chief rays are parallel to the optical axis, in object and/or image space.
Another way of describing telecentricity is to state that the entrance pupil and/or exit pupil of the system is located at infinity. If the entrance pupil is at infinity, the lens is object-space telecentric. If the exit pupil is at infinity, the lens is image-space telecentric. If both pupils are at infinity, the lens is double telecentric. For many applications, telecentricity is desirable because it provides nearly constant magnification over a range of working distances, virtually eliminating perspective angle error. This means that object movement does not affect image magnification.
A telecentric lens is a compound lens with an unusual geometric property in how it forms images. The defining property of a telecentric system is the location of the entrance pupil or exit pupil at infinity. This means that the chief rays (oblique rays which pass through the center of the aperture stop 23) are parallel to the optical axis in front of or behind the system, respectively. The simplest way to make a lens telecentric is to put the aperture stop 23 at one of the lens's focal points.
The aperture stop 23 can limit the light that traverses the optical system 2. The optical system 2 typically may have many openings, or structures that can limit the ray bundles (ray bundles are also known as pencils of light). These structures may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place, or may be a special element such as a diaphragm placed in the optical path to limit the light admitted by the system. In general, these structures are called stops, and the aperture stop 23 is the stop that determines a ray cone angle, or equivalently the brightness, at an image point.
In some contexts, the term aperture may refer to a diameter of the aperture stop 23 rather than the physical stop or the opening itself. For example, in a telescope the aperture stop is typically the edges of the objective lens or mirror (or of the mount that holds it). One then speaks of a telescope as having, for example, a 100 centimeter aperture. Note that the aperture stop is not necessarily the smallest stop in the system. Magnification and demagnification by lenses and other elements can cause a relatively large stop to be the aperture stop for the system.
In optics, an aperture is a hole or an opening through which light is admitted. More specifically, the aperture of an optical system 2 is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are, which is of great importance for the appearance at the image plane. If the admitted rays also pass through a lens, highly collimated rays (narrow aperture) will result in sharpness at the image plane, while uncollimated rays (wide aperture) will result in sharpness for rays with the right focal length only. This means that a wide aperture results in an image that is sharp around what the lens is focusing on and blurred otherwise. The aperture also determines how many of the incoming rays are actually admitted and thus how much light that reaches the image plane (the narrower the aperture, the darker the image).
The term stop is sometimes confusing due to its multiple meanings. A stop can be a physical object: an opaque part of an optical system 2 that blocks certain rays. The aperture stop 23 is the aperture that limits the brightness of the image by restricting the input pupil size, while a field stop is a stop intended to cut out light that would be outside the desired field of view and might cause flare or other problems if not stopped.
In photography, stops are also a unit used to quantify ratios of light or exposure, with one stop meaning a factor of two, or one-half. The one-stop unit is also known as the EV (exposure value) unit. On a camera, the f-number is usually adjusted in discrete steps, known as f-stops. Each “stop” is marked with its corresponding f-number, and represents a halving of the light intensity from the previous stop. This corresponds to a decrease of the pupil and aperture diameters by a factor of √{square root over (2)} or about 1.414, and hence a halving of the area of the pupil.
In optics, the f-number (also called Fno, f-stop, focal ratio, f-ratio, or relative aperture) of an optical system 2 expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the “effective” aperture diameter. It is a dimensionless number that is a quantitative measure of lens speed, an important concept in photography.
Referring to FIG. 1, in the image pickup apparatus 1, the best-focus plane coincides with the plane on which the image pickup device is disposed. FIG. 2A to 2C illustrate spot images formed on a light-receiving surface of an image pickup device 3 in the image pickup apparatus 1 shown in FIG. 1 when a focal point is displaced by 0.2 mm (Defocus=0.2 mm), when the focal point is not displaced (Best focus) or when the focal point is displaced by −0.2 mm (Defocus=−0.2 mm), individually.
An image pickup apparatus, in which light is regularly dispersed by a phase plate and is reconstructed by digital processing to achieve a large depth of field, has been suggested. Furthermore, an automatic exposure control system for a digital camera in which a filtering process using a transfer function is performed has also been suggested.
As a focusing method, a so-called hill-climbing autofocus (AF) method is known in which a focal position is determined by acquiring a peak value of contrast.
In the image pickup apparatuses shown in FIG. 1, it is assumed that a Point Spread Function (PSF) obtained is constant when the above-described phase plate is placed in the optical system 2. The point spread function (PSF) describes the response of an imaging system to a point source or point object. The degree of spreading (blurring) of the point object is a measure for the quality of an imaging system.
If the PSF varies, it can be difficult to obtain an image with a large depth of field by convolution using a kernel. Therefore, setting single focus lens systems aside, in lens systems like zoom systems and autofocus (AF) systems, there is a large problem in adopting previous structures because high precision is required in the optical design, thereby increasing costs accordingly. More specifically, in known image pickup apparatuses, a suitable convolution operation cannot be performed and the optical system 2 should be designed to eliminate aberrations, such as astigmatism, coma aberration, and zoom chromatic aberration that cause a displacement of a spot image at wide angle and telephoto positions. However, eliminating the aberrations, can increase the complexity of the optical design, the number of design steps, the costs, and the lens size. In addition, in the known image pickup apparatuses, for example, in the case where a bright object is shot, a phase of a phase modulation element varies when the aperture stop 23 is moved away from the phase modulation element.
If the aperture stop 23 is moved away from the phase modulation element while the system is not telecentric, it may distort the reconstructed image. In addition, the PSF may vary and the image reconstruction can become difficult unless the incident angle at which the light rays are incident on the image pickup device is controlled.
Accordingly, there is a need for an image pickup apparatus operable to simplify an optical system, reducing the costs, and obtaining a reconstruction image which has an appropriate image quality in which the influence of noise is small.